1. Field of the Invention
The present invention relates to a projection welding apparatus, a projection welding method, and a projection welding structure.
2. Description of the Related Art
Projection welding is known for welding a welded member to plural welding projections (projections) by passing current through the plural welding projections formed as welding portions to a welding member (base member), whilst pressing the welded member onto the welding projections, as described in Japanese Patent Laid-Open (JP-A) No. 9-57462.
In projection welding, the settings of the current passed through the welding projections are important. However, if a welding face of the welded member and a surface other than the welding projections of the welding member contact prior to completion of the welding between the welding projections and the welded member, splitting of the current occurs at the contact locations. When such splitting of current occurs, the current flowing in the welding projections decreases and the amount of heat generated is reduced, with this being a cause of poor welding such as, for example weld strength being reduced.
If all of the welding projections do not contact the welding face of the welded member at the same time, or at almost the same time, due to for example variation in the heights of the welding projections caused by manufacturing tolerance, or distortion of the welding face of the welded member, and particular welding projections contact the welding face of the welded member before others, the length of time during which current is being passed through the particular welding projections increases.
As a result, the amount of heat generated in the welding projections that contact the welding face of the welded member first becomes excessive, this being a cause of poor welding such as, for example molten material spattering.
In consideration of the above circumstances, an object of the present invention is to provide a projection welding apparatus, a projection welding method, and a projection welding structure in which contact is suppressed between a first plate member and a second plate member at locations other than the welding projections, whilst also being able to suppress molten material from spattering.